1. Field of the Invention
This invention relates to electrical submersible pumps, and more particularly to electrical submersible pumps that are powered by an electrical cable and have hydrodynamic (fluid film) thrust bearings in their motor section and in their seal section when employed.
2. Background of the Invention
Electric submersible pumps are used to lift liquids such as oil, water, or brine. They are typically long relative to their diameter to fit down drilled holes. For deep wells, a typical pump system consists of a multi-stage centrifugal pump, rotary gas separator, seal section, and motor. The motor is connected to an electric power source at the surface via electric cables. The rotor in the electric motor incorporates a tilting-pad hydrodynamic (fluid film) thrust bearing to support the rotor weight. The seal section prevents external well fluids from entering the motor and equalizes the pressure between the well fluid and internal motor lubrication oil. The seal section also houses up and down hydrodynamic tilting pad thrust bearings to react loads developed by the centrifugal pump.
For shallow wells such as those used to pump water from drilled holes for potable water, the seal and gas separator sections are eliminated. Tilting pad bearings are still used in the motor section.
For both deep and shallow well pumps, it is industry practice to manufacture the bearing pads of a relatively soft material and run them against a harder runner.
FIG. 5 illustrates the seal section portion of a known deep well submersible pumping system, produced by Baker Hughes of Houston, Tex. under the name Centrilift™. As shown, this seal section of the submersible pumping system includes a runner 550 between a down-thrust bearing 552 and an up-thrust bearing 554.
Tilting pad type hydrodynamic thrust bearings are also known from the present inventor's previous U.S. Pat. No. 4,676,668 to Ide, U.S. Pat. No. 5,137,373 to Ide, and U.S. Pat. No. 5,125,754 also to Ide, which are incorporated herein by reference in their entirety. In U.S. Pat. No. 5,137,373, the bearing can be formed in a single piece; in the other two patents, individual bearing pads are supported in a separate carrier.
In the context of a hydrodynamic thrust bearing, the stationary bearing pads face a rotating “runner” that is secured to (e.g., as by a key) or integrally formed with a shaft, in the presence of a lubricant (typically oil, but also water or a “process fluid”), which effectively separates the two components of the system via an elevated pressure developed at the interface as a result of hydrodynamic effects. For electric submersible pump seal sections, the direction of loading may be in two opposite directions. Hydrodynamic bearings are therefore spaced on both sides of the rotating runner.
In the prior art bearing/runner assemblies described in the aforementioned patents, the pads and runners are fabricated from dissimilar materials to preclude galling should the two components come into contact while under load (particularly at start-up, but also during overload conditions while running). In practice, the pads are, for example, either monolithic bronze, carbon, or fabricated from steel coated with a relatively “soft” non-ferrous material on the running surface, while the runners are typically hardened steel. Among the coatings used are engineered plastics. Industry practice is to apply the various coatings to the bearing pads.